In recent years, with the development of human clothing and various wearable portable electronic devices such as wearable computers, smart wear, etc., there has been a sharp rise in the demand for portable power supplies that can be driven for a long time and are slim and light enough to minimize the burden of carrying them.
Conventionally, as portable power supplies, chemical cells or rechargeable batteries have been used, and they can cause serious environmental pollution when they are disposed of.
To avoid this problem, devices that can use natural energy produced from wind power, geothermy, solar heat, or wave power, etc. as electric energy and can provide environmentally friendly power have been developed, but they are too big to carry. Further, since these devices are highly influenced by the environment, they have low efficiency per unit area.
Therefore, research has been conducted to develop energy harvesting techniques that can convert human kinetic energy that can be continuously generated into a new electrical power source. That is, such research is based on the idea that power supplied by utilizing energy generated from motion of the human body can be a continuous energy source that is not restricted by time and space, and thus can overcome the problems of conventional electrical power supplies.
Among the energy harvesting techniques, those utilizing piezoelectric materials have been widely studied because these materials can be operated on the basis of motions of the human body, such as a heart rate, respiration, muscle contractions, and eye movements.
For example, a flexible piezoelectric energy harvester that is in a substrate form and includes a piezoelectric material such as polyvinylidene fluoride (hereinafter also referred to as “PVDF”) has been disclosed. The piezoelectric energy harvester includes a grid-patterned piezoelectric layer, which is composed of first piezoelectric lines or islets and second piezoelectric lines consisting of a piezoelectric polymer and filling gaps between the first piezoelectric lines, and therefore is flexible and hard to break (Patent Document 1).
Meanwhile, a sandwich-formed piezoelectric fabric in which a piezoelectric layer manufactured by electrospinning is disposed between electrode layers formed by electrospinning conductive polymers has been disclosed (Patent Document 2).
Both of the conventional arts pertain to flexible two-dimensional piezoelectric energy harvesters. However, these piezoelectric energy harvesters exhibit low flexibility and elasticity and thus are difficult to fabricate into clothing or fabrics having complicated structures, and their efficiency at converting motion of the human body into electric energy is significantly low.
In order to solve both the above-mentioned problems and to manufacture a wearable energy harvester, a technique of modifying a conventional two- or three-dimensional piezoelectric energy harvester into a one-dimensional fibrous form has been suggested. Due to the high degree of mechanical freedom, such a fiber-type energy harvester could also be sufficiently used as a material for pleated fabrics.
As examples, a hybrid fiber-type energy harvester including zinc oxide (ZnO) and PVDF (Non-patent Document 1), and an energy harvester including a common axis of polyvinylidene fluoride-co-trifluoroethylene (PVDF-TrFE) and two or more fibers and a textile fabricated therewith (Non-patent Document 2) have been disclosed. However, because these two energy harvesters also employ piezoelectric ceramics, which generally serve as metallic electrodes, they still have low flexibility and elasticity, and thus are limited to the structure of cloth.    (Patent Document 1) KR patent publication 10-2014-0073201    (Patent Document 2) KR patent publication 10-2012-0083261    (Non-patent Document 1) M. B. Lee, C. Y. Chen, S. W. Cha, Y. J. Park, J. M. Kim, L. J. Chou, Z. L. Wang, Adv. Mater. 2012, 24, 1759    (Non-patent Document 2) M. B. Kechiche, F. Bauer, O. Harzallah, J. Y. Drean, Sensors and Actuators A. 2013, 204, 122